add module checks & fix notmuch module

[Lassulus]

.config made using the notmuch module a bit clunky. should be better now

[Copilot]

Pull Request Overview

This PR refactors the notmuch module configuration and introduces a check system for validating modules. The primary change renames the config option to settings in the notmuch module to avoid confusion with the standard NixOS config attribute. The PR also adds infrastructure to automatically discover and run module-level checks.

Key changes:

• Renamed notmuch module's config option to settings for clarity
• Added check files for notmuch and mpv modules to validate basic functionality
• Updated module loading to use directory-based structure with module.nix files
• Extended the flake's checks system to automatically include module-level checks

Reviewed Changes

Copilot reviewed 5 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
modules/notmuch/module.nix	Renamed config option to settings and updated references
modules/notmuch/check.nix	Added test to verify notmuch module can be applied and runs successfully
modules/mpv/check.nix	Added test to verify mpv module can be applied and runs successfully
modules.nix	Updated to load modules from directory structure expecting module.nix files
flake.nix	Extended checks to automatically discover and run check.nix files in module directories

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